1. Technical Field
The present invention relates to rotors for rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of Related Art
There are known IPM (Interior Permanent Magnet) motors that have permanent magnets embedded in a rotor core. The IPM motors can use both reluctance torque and magnetic toque, thereby achieving high efficiency. Therefore, the IPM motors are particularly suitable for use in hybrid and electric vehicles.
An IPM motor generally includes a stator and a rotor that is disposed in radial opposition to the stator. The rotor includes a rotor core and a plurality of permanent magnets. The rotor core has a plurality of pairs of magnet-receiving holes formed therein. Each pair of the magnet-receiving holes is arranged in a substantially V-shape that opens toward the stator side. Each of the permanent magnets is received in a corresponding one of the magnet-receiving holes of the rotor core. Further, for each pair of the magnet-receiving holes, the two corresponding permanent magnets which are respectively received in the two magnet-receiving holes of the pair are arranged so as to together form one magnetic pole of the rotor. In addition, for each pair of the magnet-receiving holes, there is formed a corresponding center bridge that extends in a radial direction of the rotor core between the two magnet-receiving holes of the pair to separate them from each other.
Moreover, magnetic flux leakage via the center bridges can be reduced, for example as disclosed in Patent Document 1 (i.e., Japanese Unexamined Patent Application Publication No. 2000-278896), by suitably designing magnetic flux barriers at opposite ends of each magnetic pole of the rotor.
On the other hand, Patent Document 2 (i.e., Japanese Unexamined Patent Application Publication No. 2011-211860) discloses a technique of providing relatively large magnetic flux barriers each of which extends from a magnetic pole centerline-side end of a corresponding one of the magnet-receiving holes of the rotor core toward a longitudinal axis of the rotor core, thereby reducing magnetic flux leakage toward the longitudinal axis. Moreover, by providing the relatively large magnetic flux barriers in the rotor core, it is possible to reduce the weight and thus the inertia of the rotor core. In addition, the magnetic flux barriers are arranged in pairs each of which corresponds to one pair of the magnetic-receiving holes of the rotor core.
However, according to the disclosure of Patent Document 2, for each pair of the magnetic flux barriers, the corresponding center bridge, which radially extends between the two magnetic flux barriers of the pair, has an internal barrier formed therein and is thus circumferentially divided by the internal barrier into two parts. Consequently, the total circumferential width of the corresponding center bridge is increased by the circumferential width of the internal barrier; thus it becomes necessary to offset the pair of permanent magnets received in the corresponding pair of the magnet-receiving holes of the rotor core circumferentially outward.
Further, with the circumferentially-outward offset of the permanent magnets, the circumferential width of q-axis core portions of the rotor core is accordingly reduced. Here, the q-axis core portions denote those portions of the rotor core each of which is formed between one circumferentially-adjacent pair of the magnetic poles of the rotor and through which q-axis magnetic flux flows. Moreover, the circumferential width of bridges that are formed between stator-side core portions of the rotor core and the q-axis core portions is also reduced; the stator-side core portions are located on the stator side of the magnet-receiving holes of the rotor core.
Consequently, with the reduced circumferential width of the q-axis core portions, the amount of q-axis magnetic flux which is allowed to flow through the q-axis core portions is also reduced, thereby lowering the output torque of the rotating electric machine. Moreover, with the reduced circumferential width of the bridges formed between the stator-side core portions and the q-axis core portions, it becomes easy for a magnetic short circuit to be formed within the rotor via the bridges, thereby reducing the amount of magnetic flux transferred between the rotor and the stator.